1. Field of the Invention
This invention pertains to Acousto-optical Apparatus and Systems and to Means for Determining the Chip Rate of a Pseudo-Random Signal Sequence.
2. Description of Related Art
Acousto-Optic Apparatus for radio frequency spectrum analysis are described in Chapter 4, "Applications of Acousto-Optic Techniques to RF Spectrum Analysis", of the book, "Acousto-Optic Signal Processing Theory and Implementation", edited by Norman J. Berg and John M. Lee, published by Marcel Dekker, Inc., in 1983. Chapter 9 of the same book pertains to "Incoherent-Light Time-Integrating Processors", and Chapter 10 pertains to "Coherent Time-Integration Processors". Pages 303, 306, 320-321 discuss pseudo-random signals in connection with acousto-optic apparatus.
Briefly, in an acousto-optic spectrum analyzer, a laser light is transmitted through a Bragg Cell that is mechanically modulated by a radio frequency signal. The radio frequency signal, through a mechanical transducer such as a quartz crystal, delivers a mechanical wave to the Bragg Cell, and the mechanical wave modifies the index of refraction of the Bragg Cell so that the interaction between the laser light and the mechanical wave causes a deflection of the light as a function of the frequency of the radio frequency signal. The interaction may alternatively be characterized as the interaction between the photons of the laser light and the photons of the radio frequency sonic signal. The deflection is sensed by a row of photosensors, and the energizing of a particular photosensor signal is thereby a measure of the frequency of the radio frequency. In the presence of a plurality of radio frequency signals, the laser light has a plurality of deflected beams, and a plurality of sensors are energized, signifying the precences and frequencies of such plurality of radio frequency signals. Such simultaneous frequency measurement is important in identifying the frequency content of incoming radar signals, and the like.
Signal Interrogation systems are faced with the challenge of detecting and processing low-probability-of-intercept radar signals wherein the radar systems use spectrum spreading modulations to avoid interception. Theory and the current the state of the art information indicate that a processing gain on the order of 50 to 60 decibels may be experienced by an authorized receiver to which the spectrum spreading modulation code is known, whereby the radar can reduce the density of its effective radiated power by a like amount.
Thus, detecting such emission poses two significant problems: the signals must be hunted when none of their parameters are known; the signals must then be found when they are of extremely low intensity.
Frequently masked radio frequency or radar signals carrying information and data, including radar data, are encoded onto pseudo-random signals. It is important to unmask such signals and to decode them. An information signal, together with a pseudo-random signal may be carried by a radio frequency carrier, and it may appear to be noise.
Typically the data is digital data in the form of a binary bit train having a bit-period T.sub.1, and a bit-frequency which is the reciprocal of T.sub.1. Superimposed upon that information bit train is a pseudo-random bit train having a bit-period T.sub.c, and a bit frequency called the "chip rate" which is the reciprocal of T.sub.c. The term "chip rate" will be so-used herein.
When one knows the value of T.sub.c, one can, by prior art means, extract both the carrier frequency and the encoded information from the pseudo-random background signal. When one does not know the chip rate T.sub.c, one can sense the incoming signal, delay it by a time delay .tau. and multiply the delayed signal by the incoming signal. The product of the incoming signal and the delayed incoming signal is then integrated. If the signal contains a pseudo-random pulse train, and if the time delay .tau.=T.sub.c, the period of the chip rate, the radio frequency spectrum will display signal spikes at odd multiples of .tau.=T.sub.c about the carrier frequency. However, if the purpose of the apparatus is to determine the value of T.sub.c of an incoming signal very rapidly, and particularly if the incoming signal is of short duration or is part of a radar signal, one would need a plurality of such circuits and integrators, each having a different time delay .tau.. Further, the prior art procedures are very time consuming, and time is of the essense in a threat warning system.